1. [Field of the Invention]
This invention relates to thin film transistors using a semiconductor film of crystals that are collected having various azimuths (hereinafter referred to as crystalline semiconductor film) as represented by a polycrystalline silicon film, and a semiconductor device formed by using the above thin film transistors. In particular, the invention relates to a semiconductor film forming a channel-forming region, a source region and a drain region of a thin film transistor and to a semiconductor device mounting the above thin film transistors. In this specification, the semiconductor device refers to devices that work by utilizing semiconductor characteristics as a whole inclusive of display devices as represented by a liquid crystal display device and semiconductor integrated circuits (microprocessors, signal processing circuits and high-frequency circuits).
2. [Prior Art]
There has been developed a technology for fabricating thin film transistors (hereinafter abbreviated as TFTs) by forming a crystalline semiconductor film on a glass substrate or on a quartz substrate. Application of this technology has been forwarded in a field of flat panel displays as represented by an active matrix liquid crystal display device. TFTs are used as switching elements in the pixels or as elements for forming a driver circuit formed in the peripheries of the pixels.
Silicon is chiefly used as a material of a crystalline semiconductor film for forming the channel-forming regions, source regions, drain regions or low-concentration drain (lightly doped drain: LLD) regions in active regions of TFTs. The silicon film having a crystalline structure (hereinafter referred to as crystalline silicon film) is formed by subjecting an amorphous silicon film deposited on a substrate by a plasma CVD method or a low pressure CVD method to the heat treatment or to the irradiation with a laser beam (hereinafter referred to as laser treatment in this specification).
In conducting the heat treatment, however, the heating must be effected at a temperature of not lower than 600° C. for not less than 10 hours to crystalize the amorphous silicon film. The above treating temperature and the treating time are not necessarily suitable from the standpoint of productivity of the TFTs. When a liquid crystal display device is taken into consideration as an applied product using TFTs, a heating furnace of a large size is necessary to cope with an increase in the area of the substrate, not only consuming energy in large amounts in the steps of production but also making it difficult to obtain homogeneous crystals over a wide area. In the case of the laser treatment, it is difficult to obtain homogeneous crystals due to the lack of stability in the output of the laser oscillator. Dispersion in the quality of crystals could become a cause of dispersion in the TFT characteristics, and deteriorates the quality of display of the liquid crystal display devices and the EL display devices.
There has also been proposed a technology for forming a crystalline silicon film through the heat treatment at a temperature lower than the temperatures employed thus far by introducing, into the amorphous silicon film, metal elements that assist the crystallization of silicon. According to, for example, Japanese Patent Application (Kokai) Nos. 7-130652 and 8-78329, a crystalline silicon film is obtained by the heat treatment conducted at 550° C. for 4 hours by introducing such a metal element as nickel into the amorphous silicon film.
In the crystalline silicon film formed by the above conventional methods, however, the planes of crystalline azimuth exist in a random fashion, and the ratio of orientation is low for particular crystalline azimuths. The crystalline silicon film obtained by the heat treatment or the laser treatment permits plural crystalline particles to be precipitated and oriented on {111}. Even when limited to the plane azimuth, however, the ratio of orientation did not exceed 20% of the whole film.
When the ratio of orientation is low, it is almost impossible to maintain continuity of lattice on the crystalline grain boundaries where the crystals of different azimuths abut to each other, and it is estimated that unpaired bonds are formed much. The unpaired bonds on the grain boundaries could become centers of trapping the carriers (electrons/holes) accounting for a drop in the carrier transport property. That is, since the carriers are scattered and trapped, a TFT having a high electric-field mobility cannot be expected despite the TFT is fabricated by using the above crystalline semiconductor film. Besides, since the crystalline grain boundaries exist in a random fashion, it is difficult to form the channel-forming region using crystalline particles having a particular crystalline azimuth, and electric characteristics of the TFT tend to become dispersed.